Main Rivers Basin Facilities Planning Program

Client: Allegheny County Sanitary Authority (ALCOSAN)

The Challenge

Addressing its combined and separate sanitary sewage overflow problem was a priority for the Allegheny County Sanitary Authority (ALCOSAN) and the 83 municipalities comprising its service area, which includes a population of 896,500 and 250 combined sewer overflows (CSOs). In 2008, a Consent Decree (CD) was issued with a mandate by the United States Environmental Protection Agency (USEPA) for ALCOSAN to prepare and implement a comprehensive wet weather plan to greatly reduce the annual discharge of untreated sewage into area waterways by 2026. When implemented, the Regional, Long-Term Wet Weather Control Plan (“Control Plan”) will be the largest public works project ever completed in Allegheny County. To efficiently manage the control alternatives evaluation process, ALCOSAN divided the service area into seven separate planning basins with detailed basin facility plans being developed for each. Chester Engineers was assigned as Basin Planner for the Main Rivers Basin (MRB), largest and most unique of the seven Basins as it encompasses the greater downtown Pittsburgh area which is bracketed by three large rivers. MRB’s 101 total CSO outfalls amount to nearly half of all outfalls in ALCSOAN’s service area. The MRB is over 15,000 acres, contains 14 miles of deep tunnel interceptor system, and has over 3,000,000 feet of sanitary and combined sewers ranging in sizes from 8-inches to 126-inches in diameter.

The Interdisciplinary Approach

Chester’s Main Rivers Basin Facilities Planning Program was an on-going, multi-faceted program which includes a comprehensive combined and sanitary sewer system assessment, collection system hydraulic and hydrologic modeling, and development of a facility plan to address ALCOSAN’s Consent Decree requirements. The planning project cost was estimated at $6.5 million and included various investigations of the collection system including existing capacity, projection of future flow and needed capacity and conditions assessments. Selecting potential sites for treatment and storage control facilities can be particularly challenging in downtown Pittsburgh, where historic industrial riverfronts have been replaced with a casino, two stadiums, a convention center, North Shore Light Rail Connector, and a multitude of businesses. As a result, control technologies such as deep tunnel storage and conveyance, green infrastructure, and consolidation of multiple CSO outfalls, were taken into consideration to reduce or eliminate the number of treatment and storage facilities needed.

While the project area was a large metropolitan area, the approach to developing and evaluating gray and green infrastructure alternative solutions (along with other control strategies) has direct application to small and medium sized urban areas as well. A collection system model and various Combined Sewer Overflow (CSO) control technologies was used in development of the final report.

The Sustainable Result

ALCOSAN has worked to develop an EPA-approvable Comprehensive Wet Weather Control Plan, evaluating a wide range of possible alternatives. Several key studies and reports have been prepared as necessary precursors to final basin facilities plan and regional wet weather plan. Existing sewer system characteristics were assessed and presented in the Existing Information and Conditions Report. The Hydrologic & Hydraulic Model Validation and Characterization Report established the 2003 “typical” year as the most representative hydrological period to be used as the baseline of analysis. The Report identified approximately 2,900 CSO events totaling 2.7 billion gallons of overflow volume during the typical year. A rigorous evaluation of sites and control technologies was conducted and presented in the Screening and Controls and Site Report. Potential sites to accommodate future control facilities were screened and reduced from approximately 66,000 parcels within the Main Rivers Basin to a more manageable 63 potential sites.

CSO statistics were used to evaluate the size and cost of various overflow control strategies which ranged from 0 to 20 overflows per year during the typical year of wet weather. A draft MRB Feasibility Report and Present Worth Analysis (Feasibility Report) was completed as a final step before preparing the basin facilities plans. The information presented in these reports will become incorporated into the final Basin Facilities Plan and the System Wide Wet Weather Plan.

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Project Highlights:

102 CSO facilities require a higher level of control to protect water quality.

“Green” and “gray” infrastructure alternatives were evaluated.

A new deep storage/conveyance tunnel was the preferred project.

Over 40 new drop shafts and a system of shallow conduits were required to deliver
combined sewage to the new deep tunnel.